Lack of functionality for ByteString Arrays.

[Andrew Gibbons]

The Data.Array.Repa.Repr.ByteString module provides a way to make arrays from ByteString types using fromByteString. The resulting array has the type Array B sh Word8. It seems that this type is not very useful as there is no equivalent of computeS or computeP for this representation type. Importantly, there seems to be no way to convert any other type of array to an `Array B`.

Is this implementation incomplete?